1. Field of the Invention
The present invention relates to a motor with reduction gears to be used as a power source of a general industrial machine such as a conveying machine or a manufacturing machine. The present invention especially relates to an improvement in the sealing of the grease which lubricates the gears.
2. Description of the Related Art
In a geared motor, helical gears are effective for noise reduction and are used as initial high speed reduction gears greasebath type lubrication is widely adopted to lubricate the reduction gears, to reduce noise, and to extend the life of the gears, because it can be located free from the direction of the gears and because it can be easily maintained.
A conventional geared motor having greasebath type lubrication is described referring to FIG. 7. FIG. 7 is a cross-sectional view of the conventional geared motor.
In FIG. 7, reduction gears 1 having an output shaft 1a are contained in a gear case 2. A frame 3 of the motor 50 has a hole 3a, to which a ball bearing 9 is fit, on the bottom thereof. A stator core 4 is fit to an inner face 3b of the frame 3 and fixed thereon. A helical gear 5a is formed on an end of a rotor shaft 5 and is gear-coupled with another helical gear 1b of the initial gear of the reduction gears 1. A rotor 6 of the motor 50 is fixed on the rotor shaft 5. A bracket 7 of the motor 50 is fit to the inner surfaces 2a of the gear case 2 and 3b of the frame 3. An outer wheel 8a of a ball bearing 8 is fit in a hole 7a of the bracket 7 and an inner wheel 8b of the ball bearing 8 is press-fit to the rotor shaft 5. An outer wheel 9a of another ball bearing 9 is fit in the hole 3a of the frame 3 and an inner wheel 9b of the ball bearing 9 is press-fit on the rotor shaft 5.
A spring washer 10 is provided between the ball bearing 9 and the bottom face of the hole 3a of the frame 3 for supplying a thrust pre-load to the ball bearing 9. A simplified brake mechanism 11 is provided in a side wall 3c of the hole 3a of the frame 3. The simplified brake mechanism 11 comprises: a brake shoe 11a and a spring 11b for supplying pressure to the brake shoe 11a which are provided in a hole 3d the side wall 3c; a brake disc 11c fixed on the rotor shaft 5; and a spring cap 11d. Braking force which is to be supplied to the motor 50 is made by friction between the brake disc 11c and the brake shoe 11a. The spring cap 11d is fixed on an outer face 3e of the frame 3 by a screw 12.
In such a conventional geared motor, the ball bearing 9 which is apart from the reduction gears 1 is generally smaller than the ball bearing 8 which is adjacent to the reduction gears 1, because the simplified brake mechanism 11 is provided in the side wall 3c of the hole 3a of the frame 3 to which the ball bearing 9 is to be fit.
An oil seal 13 is provided between the output shaft 1a of the reduction gears 1 and the gear case 2 for preventing the leakage of the grease 15 (which fills the gear case 2 to the outside of the gear case 2). Another oil seal 14 is also provided between the rotor shaft 5 and the bracket 7 for preventing leakage of the grease 15 to the inside of the motor 50 (or the inside of the frame 3). The oil seals 13 and 14 are made of rubber and each has a main lip 13a or 14a and a dust lip 13b or 14b. Generally, the dust lips 13b and 14b are positioned behind the main lips 13a and 14a against the material, here grease 15 being sealed. In the surroundings of the reduction gears 1, a large amount of the grease 15 is required to lubricate the gears.
The driving force of the motor 50 is transmitted to the reduction gears 1 via the rotation of the rotor shaft 5. The rotor shaft 5 is born by the two bearings 8 and 9 since the inner wheels 8b and 9b are press-fit to and fixed on the rotor shaft 5. The outer wheels 8a and 9a are respectively fit in the holes 7a of the bracket 7 and 3a of the frame 3 with gaps of 10 to 20 .mu.m separating the inner wheels and outer wheels.
The reduction gears 1 are generally made of steel because of steel's strength. The steel gears, however, are easily worn and make excessive noise when they are geared each other without oil. Accordingly steel reduction gears must be lubricated. In order to assure long life of the gears and in order to reduce noise, the above-mentioned grease-bath type lubrication is widely adopted in the conventional geared motor.
In the above-mentioned grease-bath type lubrication, a large amount of the grease 15 with a high viscosity and a inertia is used to lubricate the steel gears. The grease 15 always flows during the rotation of the gears, so that plenty of grease 15 is supplied to the output shaft 1a which rotates to transmit the driving force of the motor 50 to the outside, and so that grease 15 surges along the rotor shaft 5 which rotates to transmit the driving force to the reduction gears 1. Accordingly, the oil seals 13 and 14 are necessary to seal the gaps around the shafts 1a and 5.
In the above-mentioned conventional geared motor, the oil seals 13 and 14 are made of rubber. Accordingly there is a problem of short life of the seals 13 and 14 due to cracking of the seals, wear of the lips of the seals, or hardening of the lips caused by friction heating.
Some mechanisms which operate to shorten-life of the conventional seals 13 and 14 are.
(1) The magnitude of the flow of the grease 15 depends upon the rotation speed and/or rotation direction of the shafts 1a and 5. When rotor shaft 5 rotates at high speed, thrust is generated by the helical gears 5a and 1b, so that the flow of the grease 15 impinges the lips 14a and 14b of the oil seal 14. When the rotation direction of the rotor shaft 5 alternately changes, the direction of the flow of the grease 15 also alternately changes. Thereby, a dynamic pressure due to the flow of the grease 15 is intermittently applied to the oil seal 14. The lips 14a and 14b of the oil seal 14 is, on one hand, strongly pressed on the rotor shaft 5 by such a dynamic pressure of the flow of the grease 15 and immoderate friction force is generated between the lips 14a and 14b and the rotor shaft 5. On the other hand, the lips 14a and 14b are moved in reciprocation in the axial direction by a pressure inversion due to the flow of the grease 15.
Furthermore, when a thrust load due to the gearcoupling of the helical gears 5a and 1b is applied to the rotor shaft 5 in axial direction thereof, the rotor shaft 5 moves in the axial direction by the alternate inversion of the rotation direction. This movement is possible because there is a gap between the ball bearing 9 and the bottom of the hole 3a of the frame 3 and because the spring washer 10 is provided in the gap for supplying the pre-load to the bearing 9. When the rotor shaft 5 moves in reciprocation in the axial direction, the sealing between the rotor shaft 5 and the oil seal 14 is easily broken, since the sealing face of the oil seal 14 is designed short against the axial motion of the rotor shaft 5.
Moreover, when the rotor shaft 5 moves in reciprocation in the axial direction, the outer wheel 9a of the ball bearing 9 slides on the spring washer 10 due to the creep of the outer wheel 9a. As a result the spring washer 10 is often broken.
(2) Furthermore, metal powder generated by the wear of the gears or bearings 1c, 1d, 1e and 1f of the reduction gears 1 is mixed in the grease 15. When the metal powder reaches to the lips 13a, 13b, 14a and 14b of the oil seals 13 and 14 via the flow of the grease 15, the lips 13a, 13b, 14a and 14b are damaged and thereby causing grease 15 to leak.
(3) In order for the oil seals 13 and 14, it oil films must form on the slipping part between the shafts 1a and 5 and the oil seals 13 and 14. For this purpose, another kind of grease 16 is provided to lubricate the lips 13a, 13b, 14a and 14b of the oil seals 13 and 14. This other kind of grease 16 is different from the grease 15 fills cavities 13c, 13d, 14c and 14d in the vicinity of the main lips 13a and 14a and the dust lips 13b and 14b. It, however, is impossible in the conventional geared motor, to hold the lip-lubricating grease 16 inside cavities 13d and 14d facing to the reduction gears 1 the gear-lubricating grease 15 containing the metal powder mixes readily which the lip-lubricating grease 16.
Furthermore, the gear-lubricating grease 15 is viscous for heavy loading of the gear-coupling. It is not suitable for lubricating the minute gaps between the oil seals 13 and 14 and the shafts 1a and 5, because the necessary thin oil films are not formed by such viscous grease. Because the necessary oil films are is not formed between the oil seal 13 or 14 and the shaft 1a or 5, the oil seal 13 or 14 made of rubber slide directly on the shaft 1a or 5 made of metal. Therefore, the rubber becomes hard due to friction heat and long time friction. As a result, the gear-lubricating grease 15 leaks out.
Due to the above-mentioned reasons, oil seals 13 and 14 in the conventional geared motor grease-bath type lubrication require frequent service. Namely, those conventional geared motor, which use oil seals with only main lips 13a and 14a suffer early raise the leakage of the grease 15 in relatively early. The grease 15, which is for lubricating the reduction gears 1 flowing in the interior of the motor, flows further to the inside of the ball bearings 8 and 9 and air gaps between the rotor 6 and the stator 4. The adhesion of the viscous grease 15 on the ball bearings 8 and 9 and so on raises loss of the output power of the motor 50 and also raises the failure in start or burning of the motor 50.
For solving the above-mentioned problems, methods for increasing the interference of the main lips 13a and 14a of the oil seals 13 and 14 against the shafts 1a and 5 and for using two oil seals in series besides the above-mentioned type oil seal 13 or 14 have been proposed. Both methods, however, increase of the mechanical loss due to the friction of the oil seals and increase the volume which is necessary for the provision of the sealing members. Therefore, proposed methods cannot be adopted in small geared motors, e.g. geared motors which output several watt to several tens watts.
Generally, the frame 3 which has the hole 3a for accommodating the outer wheel 9a of the ball bearing 9, is made by aluminum because aluminum has splendid workability. However, in case that a load which is to be driven by the geared motor is largely rippled, or in case that the load is intermittently and/or frequently driven, creeps in radial direction or in the axial direction are generated on the surface of the outer wheel 9a due to the vibrations of the machine and/or the motor itself. This is because the coefficient of the thermal expansion of aluminum is larger than that of steel from which the outer wheel 8a or 9a of the ball bearing 8 or 9 is mode. Accordingly a gap is made between the outer wheel 8a or 9a of the ball bearing 8 or 9 and the inner face of the hole 7a of the bracket 7 or 3a of the frame 3, which were closely fit to each other at the beginning of the driving, under conditions of high temperature due to the driving of the motor.
Especially, the ball bearing 9 which does not receive the load directly easily creeps because the load applied to the output shaft 1a of the reduction gears 1 is magnified by the principle of a lever wherein. The ball bearing 8 serves as a fulcrum, and a force of magnified load is applied to the ball bearing 9. Furthermore, the ball bearing 9 is smaller than the ball bearing 8, and hence the ball bearing 9 is easily influenced by the creep.
When the outer wheel 9a of the ball bearing 9 slips on the inside face of the hole 3a of the frame 3, abnormal noise may occur. Or, holding of the rotor shaft 5 by the ball bearing 9 may become unstable by abnormal friction between the outer wheel 9a of the ball bearing 9 and the inside face of the hole 3a. Furthermore, the life of the ball bearing 9 is shortened by abnormal heating thereof due to the abnormal friction heat or by the mixing of the metal powder in the the grease 15. As a result, the torque of the motor 50 is reduced or the motor 50 seizes and finally burning occurs in the motor 50.
The afore-mentioned two problems, the leakage of the grease 15 and the creep of the ball bearing 9, are the weakest point of a motor 50 with reduction gears 1. The life of such an electric motor 50 is determined inconsistent with the merit of the grease-bath type lubrication directed for elongating the life of the gears.
As an example, in a small typical geared motor which outputs several watts to several tens of watts and has a square section of 60 mm to 90 mm of a side of the case of the reduction gears, this typical life time of conventionally lubricated spur gears on which grease is spread is about 5,000 hours. On the other hand, according to theoretical view point, the life of the geared motor adopting the grease-bath type lubrication and the helical gears can be elongated two times as long as that of the above-mentioned conventional. The life time, however, is actually limited to of 3,000 to 5,000 hours due to the above-mentioned problems of the leakage of the grease and the creep of the bearing.